This invention relates to a process for the photochemical conversion of light energy and storage thereof in chemical isomerizations. More particularly, it relates to the capture and catalytic conversion of photon light energy to effect reversible chemical isomerizations, and the storage of the light energy as potential energy in chemical bond strain resulting from the isomerizations. The energy thus stored is released, generally as heat upon subsequent reverse isomerization.
The storage of solar, or light energy has been accomplished in the past by permitting the impingement of photons of light upon a surface, generally a black surface. Light energy is absorbed and converted to thermal energy which is transferred to a material having thermal energy storage capacities, such as rocks or water. The thermal energy storage capacities of these materials are low, however on the order of about 50 to about 200 joules per gram.
It has been proposed that solar energy storage could be accomplished by utilizing photon or light energy to drive reversible chemical reactions in which the product or products of the light-driven reaction have a greater potential energy than the products of the reverse reaction, which potential energy may be released during the reverse reaction in the form of heat. Until the reverse reaction was initiated, however, the potential energy derived from the light energy could be stored with minimal dissipation. Among the suitable types of photochemical reactions for this purpose is the photoisomerization reaction in which the product of the light-driven reaction stores the light energy captured in the form of the potential energy of highly strained chemical bonds, such as in carbon containing ring systems.
U.S. Pat. Nos. 3,994,675 and 4,123,219 disclose the storage of light energy in the light-driven photoisomerization of substituted naphthalenes and in the light driven dimerization of substituted naphthalenes. The thermal energy storage capacities of the reactions exemplified are on the order of 50-120 calories/g (200-500 joules/g). The reactions exemplified were inhibited, however, by oxygen, particularly the photoisomerization reaction.
U.S. Pat. Nos. 4,004,571; 4,004,572 and 4,004,573 disclose the solar energy induced isomerization of organic isomerizable compounds to a high energy intramolecular strained ring structure. It is stated that such isomerizations may be aided by photosensitizers which absorb light in the visible wavelengths and then transfer energy to the isomerizable compound to induce the isomerization reaction to occur. The high energy isomer stores the solar energy in the strained ring structure until induced by heat or catalysis to revert to its lower energy isomeric form. The stored energy is released during the reverse isomerization in the form of heat. Various suitable isomerizable compounds and energy release (reverse isomerization) catalysts are listed in these patents, the disclosure of which is hereby incorporated by reference, as if written out in full, in the application.
U.S. Pat. No. 3,497,435 discloses the photoisomerization of cis, cis-dimethylmuconic acid using photosensitizers selected from aromatic hydrocarbons or ketones having triplet state excitation energy of above 55 kcal/mole and an oxygenated organic solvent containing water.
U.S. Pat. No. 3,350,291 discloses a method of preparing strained polycyclic hydrocarbons by the photoisomerization of cyclic olefins in the presence of a photosensitizer such as acetone, acetophenone or benzophenone as was known, or in the presence of a cuprous salt such as cuprous halide, nitrate or sulfate, Exemplified is the isomerization of bicyclo (2.2.1) hepta-2,5 -diene (2,5-norbornabiene) to quadricyclane in the presence of a cuprous chloride "pi" complex of 1,5-cyclooctadiene.
Although the photoisomerization of carbon-containing ring systems for the storage of solar energy in the resulting strained bonds is attractive, such as the photoizomerization of norbornadiene to quadricyclane, the reaction systems and photosensitizers previously investigated are not suitable for solar energy storage systems.
The photoisomerization of the naphthalenes, discussed above, is inhibited by oxygen, thus requiring any solar energy storage system which utilized the reaction to be completely air tight at all times.
Some organic photosensitizer compounds, such as acetophenone, do not strongly absorb light of the solar spectrum. Acetophenone, for example, absorbs light having a wavelength of 254 nm. The solar spectrum as received at the surface of the earth, however, contains only wavelengths of radiation greater than 292 nm, due to the screening effects of the atmosphere. The cuprous salt and cuprous salt complex type photosensitizing catalysts also absorb strongly in the ultraviolet region at about 254 nm, but these also absorb light weakly up to about 320 nm.
Energy transfer which is achieved with photosensitizers or photocatlysts is accomplished through the lowest excited electronic states, which states are phosphorescent in character. In organic photosensitizers, there is poor communication between the singlet manifold which absorbs energy and the triplet manifold which transfers energy. With organic photosensiters, only a small percentage of photons which are absorbed may effect the triplet state. Metal containing compounds such as organometallic complexes possess spin orbit coupling, such that a much larger percentage of photons absorbed effect excitation to the triplet state.
The cuprous salt photocatalyst system, however, is also air sensitive. Only the Cu(I) species in luminescent, that is, capable of electron excitation in response to photon "capture" and subsequent release or transfer (quenching) of the energy thus captured or absorbed. The Cu(II) species does not exhibit this phenomenon for the photoisomerization reactions discussed above. Because Cu(I) is highly susceptible to oxidation to Cu(II), the presence of air in the photoisomerizable compound/photocatalyst system is highly deleterious to the reaction desired due to resulting catalyst inactivation.
It is therefore an object of the invention to provide a process for the capture, storage and release of solar energy in the photoisomerization of carbon-containing ring systems capable of forming high energy intramolecular strained ring structures.
It is a further object of the invention to provide a process for the capture, storage, and release of solar energy utilizing photoisomerization catalysts capable of capturing solar energy and transferring the energy to a photoisomerizable compound, which catalysts are stable against thermal and oxidative deterioration.